The Impérieuse going at Full Speed. (From an instantaneous photograph.)

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The North German Lloyd Steamer Kaiser Wilhelm II.

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The indicated horse-power [called I. H.-P. for brevity], or that power developed by the engine as registered by the indicator, is not all usefully applied to the propulsion of a steamship. A large portion of it is used up in overcoming the resistance of the engine itself, as well as the necessary adjuncts of it, amounting often to thirteen per cent. Then, again, another portion is absorbed in overcoming the resistance of the propeller and its shafting; and as at present there is no accurate method of determining these portions, the net effective horse-power, or that usefully employed in propelling the vessel, can only be guessed at, or approximated to by calculations more or less abstruse. It is, however, the gross, or indicated, horse-power that has to be obtained and paid for, and that, therefore, is the element that has to be considered in practice; so that, from this consideration alone, any great increase in speed has to be very dearly paid for. Moreover, as has already been said, to admit of a higher speed the ship must be made much finer, which means that her carrying capacity for cargo and fuel has to be decreased; besides which the greater engine-power will add to the dead load, thus still further diminishing the vessel’s capability for carrying. This may be better understood by taking a steamer of moderate dimensions, and such as for many years was deemed sufficient for the Atlantic trade, say 300 feet long, 40 feet beam, and having a draft of water of 20 feet. Such a craft would have a displacement of about 4,800 tons, could steam 10 knots per hour with 1,000 I. H.-P., and carry 3,000 tons of cargo, fuel, stores, and equipment. Taking the distance to be steamed at 3,200 knots, and the consumption of fuel at 4 pounds per I. H.-P., it will be seen that the net consumption of coal is 571 tons; adding to this twenty-five per cent. for contingencies of weather, for raising steam, cooking, heating, etc., the ship would have to leave port with 714 tons of fuel and rather less than 2,300 tons of cargo, stores, etc., on board. If a steamship of similar dimensions were required to do the voyage at 15 knots, her design would have to be such that the displacement would not be more than 4,100 tons, the I. H.-P. at least 3,400, and the amount of fuel stored at the commencement of the voyage 1,618 tons. The machinery would probably have to be at least 400 tons heavier, so that the capacity for cargo, stores, etc., would now be reduced to 1,000 tons. The cost, too, would be greatly increased on account of the extra engine-power, and the expense in fuel would be more than doubled. The engine-and boiler-room staff would likewise be materially increased, while the earning power of the vessel would be less than half.

Seeing, however, that the power required for a certain speed varies with the cube root of the displacement squared, the proportion of power to tonnage will decrease considerably with the increase in the size, so that if, instead of the steamer above referred to of 4,100 tons, one were taken of 8,200 tons, the I. H.-P. for 15 knots—all other things remaining the same—would be very little more than 5,000; i. e., with a ship of twice the size the increase of engine-power is only forty-seven per cent. The carrying capacity and consequent earning power of such a boat is immeasurably more than that of the small one. The larger ship will, moreover, make better passages, and generally be much more economical in working, as the officers and crew will not very largely exceed that of the smaller vessel.

It was, however, owing to the more economical engine that advances in speed were rendered possible, and this is seen by referring back to the original ship, and supposing that instead of engines burning 4 pounds of coal per I. H.-P., it had ones consuming only 21⁄2 pounds per I. H.-P. in which case the expenditure on the voyage would be reduced from 1,618 tons to 1,004 tons; so that 600 tons more cargo could be taken and the cost of 600 tons of fuel per voyage saved. This was actually the case on the substitution of compound for old-fashioned low-pressure jet-injection engines fitted to the Cunard Company’s steamers as late as 1862, when their largest, fastest, and most improved steamer, the Scotia, was put on the service. But it was not until many years after the advent of the Scotia that such economic engines were in general use on the Atlantic, and it was only in 1874-75, when the Inman Company and White Star Company placed steamships having these engines in competition with the old-fashioned ones, that the day of the latter was gone.